1. Field of the Invention
There is substantial interest in improving the phenotypes of trees. There are a large number of commercially important timber species, such as the American timber species, redwood (Sequoia sempervirens), Loblolly pine (Pinus taeda), white spruce (Picea glauca), longleaf pine (Pinus palustris), western hemlock (Tsuga heterophylla) and Douglas fir (Pseudotsuga mengiesii), as well as others (Cheng, Plant and Cell Physiol. (1976) 17:1347-1350). The potential for modifying trees to make woody species resistant to stress, to accelerate growth, to improve wood or wood pulp qualities is of commercial importance.
Conventional plant breeding methods for trees have been very limited due to the prohibitively long reproduction cycles and high levels of heterozygosity. Therefore, the possibility of employing genetic engineering with woody species appears as an attractive alternative. However, plant cells (including woody species cells) are substantially different from other types of cells in their requirements for a transforming system. First, unlike unicellular microorganisms, the plant cells have a low rate of proliferation. Second, the plant cells are much more sensitive to their environment in relation to viability, proliferation and regeneration to plants. Third, in order to establish whether the foreign gene has been usefully integrated into the plant cell, it is necessary to establish that the regenerated plant expresses the phenotype. Finally, the plant cell has a strong rigid cell wall.
Because of the long time intervals involved between the manipulation of the plant cells and the demonstration of effective expression of the gene, it is essential that at each stage high efficiencies be achieved in transformation, cell growth, and regeneration. Furthermore, there are the additional considerations involved with matching a particular technique and the materials employed in that technique with the particular plant species. In addition, there is the further consideration of the subsequent handling of the transformed culture in order to obtain plants, namely plant regeneration from cells and callus.